I think cannot be because different numbers stands for a,r,t and s but 1 is the only number fits in the sequence because;
2x9=18 (wrong because it is not a single digit number that is necessary to complete the problem)
i think the correct problem is RATS x 4 = STAR and if it is the correct problem the equation would be ;
2178 x 4 = 8712 and therefore; R stands for 2, A stands for 1, T stands for 7 and S stands for 8
hope it helps...